Elephants and baleen whales are massive creatures that respond to exceptionally low frequency signals. Although we have many elephant and whale vocalization recordings, nearly nothing is known about their hearing. Indirect evidence from playbacks suggests hearing of proboscideans and mysticetes is similar and is tuned primarily to low and possibly infrasonic signals. This raises two interesting questions. First, these signals are emitted and perceived in two media, air and water, with radically different physical acoustic properties: 4.5-fold differences in sound speed; three magnitudes in acoustic impedance. For elephants and whale ears to have the same percept, whales must accommodate 60-fold acoustic pressures. Further, a commonly held tenet is that the upper limit of hearing is inversely correlated with body mass, implying there should be virtually no overlap in the hearing of these two taxa. The goal of this study was to determine how inner ears in these Tethytherians are structured, particularly for low frequency (LF) hearing. Computerized tomography and celloidin histology preparations were analyzed in six baleen whale (n=15) and two elephant species (n=7). The data show mysticetes have a substantially greater hearing range but that coiling and apical cochlear structures are similar, suggesting common mechanical underpinnings for LF hearing.